Although no document can be referred to, a method of the above type is disclosed by Texas Instruments, U.S.A. With the prior method a white light beam of the projector system is fed through a filter wheel, which comprises along a circular path a row of basic filter segments (or sectors) of basic colors. After passing the filter wheel the light beam is fed to a light valve device, in particular a micro mirror device. The light valve device has a matrix of minute light valves, such as micro mirrors as disclosed by U.S. Pat. No. 5,452,024, which can be controlled individually in a pulse width manner to direct the light beam or not to an optical part and then to a projection screen. During the passage of each basic filter segment by the light beam all of the light valves are controlled this way. One basic filter segment of a basic color, in particular green, of the row of basic filter segments has a reduced circumferential length. The space thus obtained is occupied by an auxiliary filter segment of the same color but having a smaller transmission factor. The light valves will receive less light from the auxiliary filter segment. As a result, with light control properties, such as the minimal switching time for said individual pulse width modulation of valves, being unchanged, a display resolution (as a number of bits grayscale per color per pixel) for said color is increased. Said auxiliary filter segment is known as grey filter. This method is known as DVE (Dark Video Enhancement).
A disadvantage of the prior art method is that the use of the auxiliary filter segment increases costs for manufacturing of the filter wheel.
In addition, the light beam has a finite dimension, for example 5 mm diameter, when passing through the filter wheel. As a result, during a transition from one filter segment to a next filter segment of a different color and/or transmission factor the light received by the light valve device is of composite color, undetermined or fuzzy. Such a transition is called a spoke. A spoke has a significant circumferential dimension, for example 11.5°. The auxiliary filter segment introduces an additional spoke, which reduces the useful part of the light integral and therefore the brightness for the segment having the additional spoke or of all segments if the reduction is distributed amongst them. A controller must take such reduction into account when modulating the light valves during projecting of the beam through the segment having the additional fuzzy spoke.
In addition, because of a reduced transmission factor of the auxiliary filter segment energy is wasted. The energy waste during the passage of the auxiliary filter segment may amount to 75%.